Conventional color video comprising intensity and color imagery is known to be unable to resolve certain objects or features in a scene, such as between backgrounds and targets in military applications. Polarization information is known to be helpful for resolving objects or features in a scene, including distinguishing between backgrounds and targets. For example, polarimetric imagery is generally needed to identify camouflage, concealment & deception (CC&D)/hidden targets because CC&D/hidden targets are not readily identified by color video alone. Polarimetric color imaging systems usually record light properties comprising color (waveband), intensity (counts) and polarization (electric field orientation).
Imaging systems can be active (include a radiation source(s), such as a laser or LED), or be passive (i.e. not include any radiation sources and thus rely on reflected or scattered naturally occurring radiation). Passive systems have the advantage of simplicity and non-detectability in defense applications.
Commercial digital cameras capture the various colors by dividing the light into individual color micro-filters. These color micro-filters are usually arranged over individual pixilated sensor elements in an array of photodetector elements of the light capture device, and generally transmit either red (R), green (G), or blue (B) wavebands. Embodied as polarimetric imagers, such cameras generally capture polarization information by adding an external rotating filter wheel polarizer that provides three or four different polarization orientations, wherein only one electric field orientation is recorded at a given time (per frame/rotation). Three to four rotations/orientations of the polarizer can span the set of linear polarizations required for most polarimetric imaging applications.
Capturing both color and multiple polarizations simultaneously poses several problems. Known configurations for simultaneous capture of color and multiple polarizations include multiple camera arrays, multiple frames, and arrangements in which part of the recorded intensity is from color and part of the recorded intensity is from polarization. Multiple camera arrays generally require two to four individual detector arrays, with three being the most common. In each case, the system cost, power and size scale with the additional detector arrays required. For multiple frames, typically three or four frames are recorded to acquire a complete set of unambiguous color and polarization signals. This reduces system performance and often increases mechanical complexity.
In arrangements in which part of the recorded intensity is from color and part of the intensity is from polarization, system designers are generally forced to accept lower performance for reduced system requirements and faster capture, since a single recorded intensity value (per pixel) has both color and polarization signal components. As a result, the respective color and polarization signal contributions is ambiguous. This reduces the reliability of either color or polarization since it cannot be deduced which provides the information received, which becomes a significant problem when the polarization signal is of interest because the polarization signal level for many objects of interest is often below 10% of the overall signal level.